Molecular investigation on the formation and transition of condensation mode on the surface with nanostructure

To investigate the wetting transition process of vapor condensation on a nanostructured surface, a series of molecular dynamics simulations are performed. With different surface wettability n* and temperature difference, the initiation of condensation and mode transitions are mapped. Compared with smooth sur-face, the condensation on the nanostructured surface can be initiated at a smaller temperature difference or weaker wettability. When n* < 0.35, the condensate overcomes the viscous force from nanopillars to form Cassie droplets, otherwise, it will immerse into the nanopillars to form Wenzel droplet or liquid film. Cassie droplet is evolved from condensate through spontaneously dewetting transition (SDT). Three modes of SDT are observed and the underlying microscopic mechanisms are discussed. For the mode I of dewetting transition process, it is presented as a single droplet spontaneously overcoming the pinning force of nanostructure to be suspended. For mode II, droplets coalesce inside the nanopillars and gradually dewet over multiple nanopillars, or the adjacent Wenzel droplets contact to form a liquid bridge on the upper of the nanostructures, promoting the SDT. For the mode III, the growing Wenzel dro-plets coalesce with the suspended Cassie droplets to complete SDT. Meanwhile, the droplet SDT process is accompanied by a decrease in potential energy, which is mainly driven by the surface tension. For a small n* and condensation rates, mode I dominates the droplet dewetting process. With the increase of surface wettability, the proportion of mode I decreases, while the proportion of mode II and mode III increases accordingly.(c) 2022 Elsevier B.V. All rights reserved.